There are a number of implementations of the famous Turing Image test out there, but this one is fully inclusive. It's simple, transparent and easy to modify because of it's object oriented construction.
The Turing Image is test to distinguish human users from computer ones by requiring the user to visually recognize a distorted image. A variation of this code is currently being used in several places at this website.
The source code consists of several files: an example index.php, a supplied database class (I recommend you replace this with your own) and the actual Turing class and TuringImage class. | 
Please contact me prior to distributing any files.